Method for applying a burn inhibitor material to a composite propellant grain



United States atent 3,493,446 METHOD FOR APPLYING A BURN INHIBITORMATERIAL TO A COMPOSITE PROPELLANT GRAIN John D. Braun and Edward M.Roy, China Lake, Calif.,

assignors to the United States of America as represented by theSecretary of the Navy No Drawing. Filed Sept. 17, 1968, Ser. No. 761,398Int. Cl. C06b 21/00 US. Cl. 149-109 1 Claim ABSTRACT OF THE DISCLOSURE Atechnique is described for providing an inhibited composite propellantgrain having maximum adhesion between the grain surface and itsinhibitor. The process comprises leaching the propellant grain to removesurface oxidizer, applying a layer of inhibitor, evacuating, thenbreaking vacuum and applying another layer of inhibitor, followed bycuring as usual.

Background of the invention Solid propellant grains are constructed ormanufactured to meet the special requirements or design of the rocketmotor, or other device in which they will be used. In some cases it isdesirable to make a grain having a comparatively long duration ofburning and to decrease the thrust by reducing the surface burning areaof the propellant grain. To decrease the burning area, and hence to slowthe rate of gas generation, select areas of the grain are covered with alayer of nonflammable or slow-burning material called an inhibitor.Solid propellant grains may be classified as unrestricted-burning grains(no inhibitor applied) and restricted-burning grains (covered withinhibitor). Materials which inhibit burning of the grain are generallyapplied by spraying, painting, dipping, pressing, or other means, to thesurface of the grain on which restricted burning is required. Solidpropellant grains are subject to some chemical changes on the surfaceand dimensional change due to variations in temperature and pressure.Often the inhibitor cracks or becomes separated from the grain duringsuch changes and uncontrolled burning occurs on the exposed surfaces.Composite propellants consist of a metal fuel, an inorganic oxidizer anda flexible binder. Adhesion of a burn inhibitor to such a grain is\difficult, largely due to the surface oxidizer which either forms apoor bond with the inhibitor, or when removed leaves voids on thesurface which greatly reduce bond area. The present invention provides aprocess which obtains maximum adhesion between the grain surface and theinhibitor, thereby overcoming the problem of uncontrolled burning of theexposed grain surface and subsequent malfunctioning of the motor.

Summary of the invention This invention relates to a process forapplying an inhibitor to any surface of a solid composite propellantgrain.

The general purpose of this invention is to provide for coating orbonding a composite propellant grain with a restrictive-burning materialby a method which is reliable, simple, and inexpensive. Another objectis to provide a restricted-burning composite grain having the inhibitorintimately bonded to the grain thereby providing maximum adhesionbetween the grain surface and the inhibitor. The method further providesa solid composite propellant grain having a burn-inhibiting coatingwhich responds to dimensional changes due to variations in temperatureand pressure.

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Description of the invention In accordance with this invention acomposite solid propellant consisting essentially of a metal fuel, aninorganic oxidizer and a flexible binder was mixed, cast and cured toform a rocket propellant grain. The grain was leached with water orother suitable solvent to dispose of any oxidizer which remained on thesurface of the grain. After drying, the leached grain was heated to 60C. and a layer of uncured inhibitor was applied to its surface thickenough to fill all the surface cavities or pores left after the leachingand to provide a smooth outer layer of the inhibitor. The coated grainwas then placed in a vacuum at 60 C. and 0-10 mm. pressure for 10minutes, then the vacuum broken. Additional inhibitor was finallyapplied to the grain surface and the grain was heated until theinhibitor was cured.

In this new technique the surface oxidizer, if any, is first washed outby leaching which eliminates one reason for a weak bond. However, the'porous surface that results is still difficult for an inhibitor to bondto because the surface cavities or pores are largely unfilled by theinhibitor material and bonding area is lost. By heating to a temperaturesufi'icie'nt to reduce the viscosity of the inhibitor used and pullingvacuum on the leached, freshly coated grain, air is removed from theinhibitor-coated surface and when vacuum is broken, atmospheric pressuredrives the inhibitor into the cavities left by the surface' oxidizerafter leaching. This greatly increases the bond area and consequentlythe maximum bond results.

The composite propellants to which this method particularly applies areformed of a preponderant amount of solid particles of an inorganicoxidizer and a fuel uniformly distributed throughout a matrix or binder.Those oxidizers generally used include the nitrates, chlorates, andperchlorates of the alkali metals, alkaline earth metals and ammonia.Fuels include aluminum, magnesium, beryllium, and others. Suitablebinders include the synthetic rubbers based on the copolymers ofpolybutadiene with acrylic acid, methacrylic acid, vinylidene chlorideor the like, the chemical rubbers of the polyurethane type and mixturesof fluorocarbons such as polyte'tr-afiuoroethylene (Teflon) and thecopolymer of vinylidene fluoride and perfluoropropylene (Viton). Thecomposite propellant compositions also normally contain combustionmodifiers or burning rate catalysts such as sodium fluoride,ferrocyanides, magnesium oxide, chromates and the like.

Included among the many inhibitors successfully used were thecommercially available' flexible epoxy resins which contain as basicingredients about 20 parts by weight modified diglyci-dal ether ofbisphenol A type epoxy resin and about parts by weight polymericamido-amine hardener. The flexible' polyurethanes are also readilyuseable as inhibitors. The binder matrix may also be used as aninhibitor. Any suitable inhibitor will give maximum bonding to thecomposite grain surface by use of the process described herein becausebond strength is increased due to the increased bond area for theinhibitor.

The invention herein described may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. A method for applying an inhibitor to any surface of a solidcomposite propellant grain, which comprises:

(a) providing a cured composite propellant grain consisting essentiallyof a fuel, an inorganic oxidizer and a binder;

(b) leaching said grain to remove any oxidizer which may remain on thesurface of said grain, leaving a porous surface on said grain;

(c) heating said leached grain;

(d) coating the surfaceof the heated grain with a layer of liquid,uncured burn inhibitor material thick enough to fill all of said poresand form a smooth outer surface;

(e) evacuating to 0-10 mm. pressure to remove all air bubbles from thepores in the propellant surface, While maintaining a temperaturesufiicient to reduce the viscosity of said inhibitor;

(f) breaking vacuum to drive said inhibitor into said pores;

(g) applying additional inhibitor to said coated surface thereby forminga smooth outer surface; and

(h) heating until said inhibitor is cured.

References Cited UNITED STATES PATENTS BENJAMIN R. PADGETT, PrimaryExaminer 0 S. J. LECHERT, JR., Assistant Examiner S. Cl. X.R.

